The present invention generally relates to side tone correction circuits and more particularly to a side tone correction circuit for correcting the side tone characteristic of the two-wire/four-wire conversion circuit used in a facsimile apparatus which is combined with a telephone unit.
In the communication circuits for exchanging information via a telephone network, audio frequency signals are sent to and received from a transmission line or telephone line of the network via a transformer. In such a communication circuit, it is essential that an impedance matching be established between the circuit and the network in order to obtain a satisfactory communication performance.
In the case of a facsimile apparatus and the like which is equipped with a telephone unit, there is usually provided a tranformer for connection to the telephone line, and a two-wire/four-wire conversion circuit is connected to the secondary side of the transformer for amplifying and sending out the transmission signal to the telephone line and further for amplifying the received signal and supplying the same to the hand set of the telephone unit. In such a two-wire/four-wire conversion circuit, there occurs rather frequently a problem of "side tone" wherein the transmission signal to be transmitted is fed back and mixed to the reception side system as a side tone. This problem occurs particularly when the impedance matching between the telephone network and the conversion circuit is not properly established. As the condition of impedance matching changes depending on the distance between the telephone exchange station and the site where the individual facsimile apparatus is used, the level of the side tone decreases when the facsimile apparatus is connected to a distant station referred to hereinafter as "distant end" and the level of the side tone increases when the facsimile apparatus is connected to a near station referred to hereinafter as "near end". In order to compensate for these variations of side tone level, various side tone compensation circuits have been proposed so far.
For example, the Japanese Laid-open Patent Application No. 59-95758 describes a side tone correction circuit for a telephone unit, wherein there is established a condition to cancel out the side tone with respect to the distant end and an increase of the side tone level beyond the allowable level is prevented by adjusting the sound volume of the reception signal on the basis of detection of the current flowing through the network such that the sound volume level when there is a connection to the distant end has a level substantially identical to the sound volume level when there is a connection to the near end. Further, there is also a known circuit wherein a switching circuit having a hysteresis characteristic is used. In this circuit, the switching circuit is controlled in response to the detected line current. Thereby, an automatic and stable level adjusting of the received signal is achieved.
Although the foregoing prior art side tone correction circuits are effective in suppressing the side tone by adjusting the sound volume of the received audio signal, such a prior art circuit cannot provide compensation for the sound quality. Because of this, there arises a problem such that the quality of the received audio signal is deteriorated.
More specifically, the transformer has its own frequency characteristic which changes in response to the change of impedance of the telephone network connected to its primary side. Thus, although the level of the side tone can be suppressed by the adjustment of the amplification of the transmitted and received audio signals, there still remains a problem of deterioration of the sound quality of the received audio signal. Moreover, in the case of the telephone unit connected to the facsimile apparatus, the telephone unit is connected to the secondary side of the transformer and cannot detect the current in the line.